Tuning a Drilling Assembly Like a Violin
Methods to mitigate downhole vibrations is part of many successful drilling programs.  Stabilizer placement in the Bottomhole Assembly (BHA) can be adjusted for smooth drilling at a targeted rotary speed range, just as a musician must tune an instrument to make good music.  Tuning an instrument and a drilling assembly both require proper attention to the nodal points.  A tuned assembly drills faster, extends bit life, reduces tool failures, and provides improved borehole quality.  

BHA designs are evaluated for their lateral dynamic response using calculated vibration indices, enabling comparison of the relative response of different assemblies to the same excitation.  Shortening a span between contacts increases the rotary speed sweet spot range, just as shortening the length of the string on an instrument increases the pitch.  Conversely, increasing the spacing results in a sweet spot at a lower speed range.  BHA designs with MWD tools often require special attention as many of these tools have blades that make contact with the borehole. 

Many drilling tools are designed and selected independently, without proper regard for tool placement and the response of the resulting BHA.  Drilling operations may then suffer from BHA designs that have poor nodal point configurations.  Several field examples are discussed in which improved designs were selected by identification of a contact spacing issue and adjusted using a lateral vibration model, with improved drilling results.  The basic principles of BHA Redesign are used to guide the redesign process to achieve a vibrationally-quiet design and meet other BHA design objectives.